The present invention relates to a television signal scrambling method and apparatus for use in a CATV system in which a television signal is sent from a center station to a large number of terminals connected to the center station through cables.
In general, CATV systems are constructed such that programs produced in a center station or programs put on the air are transmitted to terminals through cables.
In such CATV systems, fees are imposed on part of the programs produced in the center station. For example, a viewer on each terminal side is charged a predetermined fee when he watches a chargeable program.
In order to prevent illegal watching of chargeable programs, a scrambling technique is employed on the center side to spoil television signals of chargeable programs so that usual television receivers cannot reproduce normal television pictures.
On the other hand, a descrambler for returning scrambled television signals to their original signals is provided on the side of subscribers who are permitted to watch chargeable programs.
Various methods have been proposed as the aforementioned scrambling and descrambling techniques. Among those methods is a gated sync suppression (GSS) method in which a television signal is scrambled by compressing horizontal sync signal portions and vertical blanking interval (VBI) portions of a television signal by several dBs so that the horizontal and vertical synchronization cannot be established in usual television receivers, and permitted terminals descramble the scrambled television signal by expanding the compressed horizontal sync signal portions and VBI portions to the original levels. In another method, only horizontal sync signal portions are compressed/expanded.
FIGS. 6(a)-6(c) show signal waveforms each including a VBI portion continued from the last horizontal sync signal portion, and illustrate how the scrambling is performed by the GSS method.
FIG. 6(a) shows a normal video signal after modulated to an intermediate frequency video signal IFv. In the center station, horizontal sync signal (H) portions and VBI portions of the intermediate frequency video signal IFv are compressed by a predetermined amount, for example, about 6 dB, as shown in FIG. 6(b).
Further, in the center station, key signals K indicating the timing of compression of the horizontal sync signal portions are superposed on an intermediate frequency audio signal IFa. The resultant intermediate frequency audio signal IFa and the compressed intermediate frequency video signal IFv are combined and converted to an RF signal to be transmitted to the respective terminals.
On the other hand, in each terminal, the intermediate frequency audio signal IFa is detected from the RF signal received. Further, the key signals K are detected from the signal IFa. Then, the digitally compressed horizontal sync signal portions in the video signal are expanded by about 6 dB, corresponding to the compression amount in the transmitter side, on the basis of the timing of the detected key signals K. At the same time, the VBI portions are expanded by about 6 dB, corresponding to the compression amount in the transmitter side by detecting the absence period of the key signal which corresponds to the VBI. Thus, the process for returning the compressed signal to the original television signal is completed.
However, in the case of the television signal scrambled by the aforementioned scrambling method, it is still possible for illegal CATV terminals to reproduce television pictures relatively easily by descrambling the television signal.
That is, the operation of descrambling the horizontal sync signal portions can be made easily if the timing relationship between the key signal K superposed on the intermediate frequency audio signal IFa and the digitally compressed horizontal sync signal portion in the video signal can be found. Further, the VBI portions can be descrambled by detecting the VBIs in which the key signals are not superposed successively.
To prevent the illegal reproduction of television pictures, a plurality of timing relationships between the digitally compressed horizontal sync signal portion of the video signal IFv and the key signal K superposed on the audio signal IFa may be provided, and the timing relationship may be varied with lapse of time, to thereby make it impossible to descramble the television signal by the aforementioned illegal method.
On the other hand, the timing information is encoded to produce timing mode data. For example, the timing mode data is superposed on the intermediate frequency audio signal IFa at a prescribed position, and transmitted to the regular descramblers. in each descrambler, the timing mode data is decoded and the expansion timing is controlled so as to become identical to the compression timing on the center side.
Recently, however, an apparatus of descrambling a television signal illegally by detecting the compressed portions from the scrambled video signal itself without use of the key signals K has been proposed as described in the article, "UNIVERSAL DESCRAMBLER", Radio Electronics, May, 1990.
FIG. 7 is a block diagram showing the conceptual construction of such an apparatus.
A television signal received through a cable 1 is converted, by a frequency converter 2, into a signal having a frequency of an empty channel (e.g., a second channel) which has no on-air television signal in the area of descrambler installation. The signal is applied to both a descrambling circuit 3 and a video detector circuit 4a. The video signal detected by the video detector circuit 4a is applied to a 3.58 MHz timing generator 4. In the timing generator 4, an output synchronized with color burst signals contained in the video signal is extracted.
As is well known, the color burst signal exists just after the horizontal sync signal. Accordingly, when the frequency and phase of the color burst signal is recognized, the phase of the video signal compressed on the center side, that is, in the scrambler, can necessarily be recognized on the basis of the relationship: 3.58 MHz (color burst frequency).times.2/455=15.74 kHz (horizontal sync frequency).
The lock output of the timing generator 4 is applied to a sync generator 5. For example, the sync generator 5 is constituted by a frequency-divider-type generator used in video cameras or the like. Horizontal sync reset and vertical sync reset switches 6 and 7 are provided in association with the timing generator 4 and the sync generator 5, respectively. When the screen of a television receiver is watched under the condition that both the switches 6 and 7 are on, a vertical bar caused by the horizontal blanking and a horizontal bar caused by the vertical blanking are observed as shown in FIG. 8(a). The vertical bar caused by the horizontal blanking moves left or right, and the horizontal bar caused by the vertical blanking moves up or down. If the horizontal sync reset switch 6 is turned off at the point of time when the vertical bar reaches the left or right end of the screen, only the horizontal bar caused by the vertical blanking remains in the screen as shown in FIG. 8(b), and moves up or down. When the vertical sync reset switch 7 is further turned off at the point of time when the horizontal bar reaches the top or bottom end of the screen, the timing of compression of the television signal can be set to obtain normal television pictures as shown in FIG. 8(c).
Then, a timing generator 8 operates on the basis of the aforementioned set timing, so that a control signal is fed from the timing generator 8 to the descrambling circuit 3. The descrambling circuit 3, receiving the control signal from the timing generator 8, controls a switch SW suitably so that a contact a is selected during periods of the horizontal sync signal portions and VBI portions to just pass the signal, and that a contact b is selected during periods of the other portions to feed the signal to an output terminal 9 through, e g., an 6 dB attenuator 31 having an attenuation amount equal to the compression amount on the center side. Thus, the descrambled, i.e., original television signal can be obtained at the output terminal 9.
As described above, there exist descramblers which can analyze the video signal itself scrambled by the GSS method, and can descramble it.